The present invention relates to a protective device for an induction heating apparatus used for cooking by utilizing heat from electromagnetic induction. The induction heating apparatus heats by electromagnetic induction with high efficiency without noise even though the exciting current is of low frequency such as commercial frequency.
The induction heating apparatus comprises an excitor forming at least two groups of magnetic circuits which include excitation cores wound by excitation windings. A capacitor having a specifc value is connected in series to at least one of the excitation windings so as to pass a current having a 45.degree. phase gain from the power voltage to the excitation winding and so as to pass a current having a 45.degree. phase delay from the power voltage to the other excitation winding. A heating element comprises a specific cooking apparatus e.g. a specific pot which has a bottom of superposed plates (or alloy) made of non-magnetic metal e.g. copper and ferromagnetic metal. The water or food in the specific cooking pot is heated by the eddy current loss generated by the alternating magnetic flux which is passed from the excitation core through the non-magnetic metal to the magnet metal and is returned to the excitation core through the non-magnetic metal.
In the induction heating apparatus utilizing low frequency, the current passing through the circuit which connects the excitation winding and the capacitor in series is increased and the phase is shifted when the apparatus is used in an abnormal state such as where the cooking pot is heated while empty or such as where the pot is different from the specific pot required or such as where no pot is put on the excitor. Therefore, a current or voltage higher than the rated current or voltage is applied to the excitor. This can cause burning or insulation damage. If the pot is heated while empty, a fire may result. If a pot other than the required pot is used, abnormal vibration will be caused.